1. Field of the Invention
This invention relates to a non-contact IC card and a non-contact IC card reader/writer arranged to transmit data through electromagnetic waves or the like. This invention also relates to a method of transmitting data in a non-contact manner.
2. Description of the Related Art
FIG. 10 shows a conventional data transmission system using a non-contact IC card 2. A non-contact IC card reader/writer 1 is connected to an external unit 3, and the non-contact card 2 is positioned in the vicinity of the reader/writer 1. The external unit 3 sends to the reader/writer 1 commands relating to signal transmission and receives results of communication with the IC card 2 and other signals. The reader/writer 1 has a transmitting antenna 11 for transmitting electromagnetic waves and a receiving antenna 13 for receiving electromagnetic waves. A transmitter circuit 12 is connected to the transmitting antenna 11 while a receiver circuit 14 is connected to the receiving antenna 13. A (universal asynchronous receiver transmitter) UART 15 is connected to the transmitter circuit 12 and the receiver circuit 14, and a control circuit 16 is connected to the UART 15. The control circuit 16 is connected to the external circuit 3 through an input/output circuit 17.
On the other hand, the IC card 2 has a transmitting antenna 21 and a receiving antenna 23 as in the case of the reader/writer 1. A transmitter circuit 22 is connected to the transmitting antenna 21 while a receiver circuit 24 is connected to the receiving antenna 23. A UART 25 is connected to the transmitter circuit 22 and the receiver circuit 24, and a control circuit 26 is connected to the UART 25.
The reader/writer 1 receives data from the IC card 2 as described below. Electromagnetic waves transmitted from the transmitting antenna 21 of the IC card 2 are received by the receiving antenna 13 of the reader/writer 1, and a received signal S1 is supplied to the receiver circuit 14. FIG. 11 shows blocks of the receiver circuit 14. The received signal S1 is amplified by a high-frequency amplifier circuit 141 in the receiver circuit 14, demodulated by a detector circuit 142, and amplified by a low-frequency amplifier circuit 143 as serial data S2 which is sent to the UART 15. Serial data S2 is converted into parallel data S3 by the UART 15, and parallel data S3 is supplied to the control circuit 16. The control circuit 16 reads the content of parallel data S3 and performs required processing.
The reader/writer 1 transmits data to the IC card 2 as described below. Parallel data S3 is supplied from the control circuit 16 to the UART 15 and converted into serial data S4 by the UART 15. The converted data is sent to the transmitter circuit 12. FIG. 12 shows blocks of the transmitter circuit 12. A clock signal generated by an oscillator 124 is converted into a carrier clock signal S5 having a carrier frequency by a frequency divider 123 or a multiplier. The carrier clock signal S5 supplied from the frequency divider 123 is modulated in a modulator circuit 122 with serial data S4 from the UART 15. The modulated signal is amplified by a power amplifier circuit 121 and is thereafter sent to the transmitting antenna 11 as a transmitted signal S6.
A modulation method using amplitude shift key (ASK) modulation is ordinarily used for the above-described data transmission/reception. In the ASK modulation, "0" and "1" of data are represented by the existence and non-existence of electromagnetic waves W1, as shown in FIG. 13. Actually, however, induced electrical currents do not attenuate instantly when data is changed from "0" to "1", because of occurrence of free vibration in the transmitting antenna 11. For example, transmitted electric waves W2 ring for a time period T1 in the transmitting antenna 11 after data has been changed from "0" to "1". Ringing also occurs when electromagnetic waves are received. Even if ideal electromagnetic waves W1 shown in FIG. 13 are received, received waveform W3 exists through a time period T2 after electric waves W1 have ceased to exist, because ringing occurs in the receiving antenna 13. Ringing also occurs in the IC card 2 as well as in the reader/writer 1.
Generally, ringing continues through a longer time if electromagnetic waves are stronger, and attenuates in a shorter time if electromagnetic waves are weaker. In the conventional transmission system, the intensity of electromagnetic waves output from each of the reader/writer 1 and the IC card 2 is constant and the intensity of electromagnetic waves received by the reader/writer 1 or the IC card 2 changes depending upon the distance between the reader/writer 1 and the IC card 2. With the change in the intensity of electromagnetic waves, the period of time through which the received waveform exists is changed. That is, if the IC card 2 is placed close to the reader/writer 1, each of the IC card 2 and the reader/writer 1 receives stronger electromagnetic waves, and ringing occurs for a longer time in the receiving antenna 23 of the IC card 2 or the receiving antenna 13 of the reader/writer 1. Thus, it is difficult to normally modulate or demodulate signals and there is a possibility of failure to accurately transmit data